1. Technical Field
The present invention relates to a plated material, a method of producing the plated material, and an electrical/electronic part using the plated material. More specifically, the present invention relates to a plated material that has high heat-resistance and is suited to be a material for a connector used in a high temperature environment such as an engine room of an automobile. Further, the present invention relates to a plated material that has both high heat-resistance and good insertability/extractability, so that it is suited to be a material for a fitting-type connector or contactor used in a high temperature environment.
2. Prior Art
A plated material comprising an electrically conductive base of Cu or Cu alloy and a coating of Sn or Sn alloy formed on the base is known as a high-performance electrical conductor having high electrical conductivity and high strength of the base as well as good electric-contact property, high corrosion-resistance and good solderability of Sn or Sn alloy. The plated material of this type is used widely for various terminals, connectors, etc.
As the plated material of this type, usually a material that is produced by forming an undercoating of Cu or Ni on a base and then forming a coating of Sn or Sn alloy directly on the undercoating is used. The undercoating is provided to restrain a component of the base (component of alloy such as Cu or Zn) from diffusing into the top-coating of Sn or Sn alloy. Especially when the undercoating is a coating of Ni or Ni alloy, it is highly effective in retarding the above-mentioned diffusion into the top-coating of Sn or Sn alloy even in a high temperature environment. As a result, the properties of Sn or Sn alloy of the top-coating are maintained for a long time.
However, even the above-described plated material having an undercoating of Ni or Ni alloy has a problem. That is, when the plated material is used at a place where the temperature becomes very high, for example, near an engine in an engine room of an automobile, Cu of the base and Ni or Ni alloy of the undercoating still diffuse toward the top-coating with time. After a certain time has passed, the top-coating is no longer the original coating of Sn or Sn alloy, that is, the top-coating of Sn or Sn alloy practically disappears. As a result, the plated material does not exhibit its original performance.
The problem like this can be solved by making the thickness of the top-coating of Sn or Sn alloy larger so that it may take longer for the top-coating to disappear. However, the solution like this leads to waste of resources. In addition, it may cause another problem. That is, in the case where the plated material is used for, for example, a connector where many terminals are fitted at the same time (a fitting-type connector), the above solution may make it difficult to fit the terminals to a partner member.
In the fitting-type connector, a male terminal is fitted in a female terminal to thereby form electrical connection. In recent years, regarding a connector terminal used in an automobile, transmitted information has been increasing and electronic control performance has been developing. With this, multiplication of connector pins has been proceeding. In that case, if force required for inserting a terminal stays the same, a connector having a larger number of pins needs as much larger force for insertion. Thus, regarding a connector having a large number of connector pins, reduction in the force required for insertion is demanded.
As a terminal that meets this demand, there is, for example, a terminal having a top-coating of Au. When this terminal is used, the force required for insertion reduces. However, Au is expensive, which causes another problem that the cost of producing the terminal is high.
As a connector terminal, a terminal comprising an electrically conductive base of, for example, Cu and an Sn coating formed on the surface of the base is generally used. In the case of this terminal, since Sn is a material that is easily oxidized, a hard skin layer of Sn oxide is always formed on the surface of the terminal when the terminal is in the atmosphere.
When this terminal is inserted, the hard skin layer of Sn oxide breaks at the time the terminal fits in a partner member. As a result, the non-oxidized Sn coating under the hard skin layer of Sn oxide comes in contact with the partner member, so that electrical connection is formed between both. However, if the formed Sn coating is thin, the oxide layer does not easily break when the terminal fits in the partner member. In addition, in the case where the base is of Cu or Cu alloy, Sn of the thin Sn top-coating reacts with a component of the base in practical use in a high temperature environment, so that Cu is exposed at the surface and a layer of Cu oxide is formed on the surface. As a result, reliability of contact with the partner member is lost.
The probability that the problem as above happens can be reduced by making the Sn top-coating thicker. However, this causes another problem that larger force for insertion is required when the terminal is fitted to the partner member.
Thus, there is a problem that particularly in a high temperature environment, there is no choice but to use an expensive Au-plated terminal or an Sn-plated terminal having a thick Sn top-coating and a small number of pins.
When a coating of Sn or Sn alloy is formed on the surface of a terminal, bright Sn plating or reflow Sn plating is applied generally.
In the case of a coating formed by bright Sn plating, the coating contains a large amount of additives used in plating. In addition, the grain size of Sn crystal in the coating is fine. Therefore, the surface of the coating has good lubricity, and the amount of the coating scraped off at the time of fitting or sliding is small. Thus, the coating has good insertability/extractability. However, because of the fine grain size, when the material with this coating is used in a high temperature environment, the rate of grain-boundary diffusion of a component of the base is high, so that the component of the base may diffuse up to the surface of the terminal. Thus, the material with the coating formed by bright Sn plating has low heat-resistance.
In reflow Sn plating, after plating of the entire surface is finished, the top-coating is heated and fused. As a result, in the top-coating formed by reflow Sn plating, Sn has a large grain size, and the additives that had come into the coating during plating have been removed. Therefore, even in a high temperature environment, the rate of grain-boundary diffusion of a component of the base is low. Thus, the material with the coating formed by reflow Sn plating has high heat-resistance. However, because of the large grain size, the amount of the coating scraped off at the time of fitting or sliding is large. In addition, since the amount of additives contained in the coating is small, the coating is worse in lubricity, and therefore worse in insertability/extractability.
In this situation, various methods have been proposed for improving heat-resistance and insertability/extractability of the Sn coating.
For example, Japanese Unexamined Patent Publication No. Hei 8-7940 and Japanese Unexamined Patent Publication No. Hei 4-329891 disclose methods in which a coating of a metal having a high melting point, especially of Ni is formed as an undercoating for an Sn coating so as to improve heat-resistance. In the case of these methods, in the temperature range of about 100xcx9c120xc2x0 C., the Ni coating restrains reaction between a component of the base (component of an alloy such as Cu, Zn or the like) and Sn of the Sn coating. In addition, the rate of reaction between Ni and Sn is low. Therefore, the heat-resistance effect is obtained. However, in a high temperature environment of 140xc2x0 C. or higher, the rate of reaction between Ni and Sn becomes higher, and the quality of the Sn top-coating changes. As a result, the heat-resistance effect is not obtained.
Japanese Unexamined Patent Publication No. Hei 11-121075 and Japanese Unexamined Patent Publication No. Hei 10-302864 disclose methods in which the thickness of an Sn top-coating is made small so as to improve insertability/extractability.
In the case of the Sn top-coating formed by these methods, the amount of the top-coating scraped off at the time of fitting or sliding is smaller, and insertability/extractability is better. However, since the thickness of the Sn coating is small, only with a little heating, the Sn top-coating turns into an alloy by a component of the base diffusing in it, and therefore disappears. This leads to increase in contact-resistance between a terminal and its partner member.
As stated above, with the conventional plated materials having an Sn top-coating, there is a problem that it is very difficult to ensure both heat-resistance and insertability/extractability.
An object of the present invention is to provide a plated material having a top-coating of Sn or Sn alloy which is designed to ensure that even in a high temperature environment, the rate of diffusion reaction between the top-coating and a base or an undercoating is low so that the plated material may have high heat-resistance, and also provide a plated material which has both high heat-resistance and good insertability/extractability and is suited to be a material for a fitting-type connector or contactor used in a high temperature environment.
Another object of the present invention is to provide a method of producing the above-mentioned plated material, and to provide an electrical/electric part, for example, a fitting-type connector or contactor using the above-mentioned plated material.
In order to attain the above objects, the present invention provides
a plated material comprising an undercoating of any one of metals belonging to group 4, group 5, group 6, group 7. group 8, group 9 or group 10 of the periodic table or an alloy containing any one of those metals as a main component, an intermediate coating of Cu or a Cu alloy, and a top-coating of Sn or an Sn alloy, the undercoating, the intermediate coating and the top-coating being formed on a surface of an electrically conductive base in this order.
In this case, there are provided a plated material in which it is desirable that the thickness of the undercoating is 0.05xcx9c2 xcexcm and the thickness of the intermediate coating is 0.01xcx9c1 xcexcm, and a plated material in which it is desirable that the thickness of the top-coating is 1.9 times or more the thickness of the intermediate coating.
Further, the present invention provides
a method of producing a plated material, wherein on a surface of an electrically conductive base, an undercoating of any one of metals belonging to group 4, group 5, group 6, group 7, group 8, group 9 or group 10 of the periodic table or an alloy containing any one of those metals as a main component, an intermediate coating of Cu or a Cu alloy, and a top-coating of Sn or an Sn alloy are formed in this order.
In this case, it is desirable that after the intermediate coating is formed, an Sn coating and a coating made of at least one metal chosen from a group consisting of Ag, Bi, Cu, In, Pb and Sb are formed on the intermediate coating in this order, and then reflow treatment or thermal diffusion treatment is performed.
Further, the present invention provides electrical/electronic parts, more specifically, a fitting-type connector, a contactor, etc. using the above plated material.